1. Field of the Invention
The present invention relates to a rod antenna, and more particularly to an extensible rod antenna for automobiles, etc. equipped with waterproofing structure at the joints for respective rods, etc.
2. Description of the Invention
Generally, as shown in FIG. 4, an extensible rod antenna is designed to encase a rod antenna element 1 in a cylindrical housing 2 such that the rod antenna element 1 may be optionally pushed into or pulled out from the cylindrical housing 2. This rod antenna element 1 includes a plurality of rods 1a, 1b, 1c . . . which are respectively different in their diameters and are made of, for example, BST. These rods are coupled to each other in a freely slidable manner. At each of the joints for mutual coupling of the rods of the antenna element 1, waterproofing structures are provided as shown in FIG. 5.
FIG. 5 is an enlarged sectional view of the portion A in FIG. 4 showing a smaller diameter rod 10 and a larger diameter rod 20. The inner circumferential surface at the joint area of the larger diameter rod 20 is processed with a cut-out. In this machined cut-out area, a metal O-ring 23 is seated such that it is inserted and held between first and second collars 21 and 22 which are formed into a ring shape. An open end 24 of the large diameter rod 20 is retracted.
In the conventional rod antenna described above, since the cut-out work needs to be done on the inner surface of the rods, it requires more manufacturing steps, resulting in increasing the cost of the antenna. Besides, the difference in the dimensions of the inside and outside diameters of the antenna rods results in wear due to sliding friction between the O-ring 23 and the smaller diameter rod 10, and along with other factors, it causes watertightness to be lost in a relatively short period of time. Thus, sufficient and long lasting sealing cannot be expected.
Furthermore, if the antenna rods are made of SUS, not only is the material hard in quality, but the rod wall is also in general thin. Thus, cutting work on such a rod is extremely difficult to perform. Due to the above mentioned reason, an alternative is to form a bulged-out portion on the rod coupling area in order to fit the O-ring 23 in this bulged-out portion. However, this method also increases manufacturing steps and the cost of the antenna.
In some cases, in place of the metal O-ring 23, a rubber O-ring is used; however, the rubber O-ring is low in heat resistance as well as abrasion resistance. Thus, it cannot withstand long term use. In an extreme case, the O-ring is forced out of the antenna rod. Accordingly, it has been a practice to use many parts, together with grease seals, etc. in order to accomplish waterproofing.